Not applicable.
The present invention generally relates to floating offshore mineral exploration and production platforms and, more particularly, is concerned with a compliant guide for protecting the buoyancy cans and components of the floating offshore platform from damage from impacts which occur as a result of hydrodynamic loads (e.g. Froudexe2x80x94Krylov impact forces) on the buoyancy cans.
The spacing between the buoyancy can outer wall and the contact point of the guide structure in the centerwell of a Spar type floating offshore mineral exploration and production platform has been found to be very important in determining loads on the buoyancy can. The buoyancy can will have contact points (most typically four to six), in the form of built-up wear strips. These contact points on the buoyancy can will face corresponding contact points on the guide structure. See U.S. Pat. No. 4,702,321 to Edward Horton for xe2x80x9cDrilling, Production, and Oil Storage Caisson for Deep Waterxe2x80x9d and U.S. Pat. No. 4,740,109 to Edward Horton for xe2x80x9cMultiple Tendon Compliant Tower Constructionxe2x80x9d, both incorporated herein by reference.
Although sensitivity to gap size had previously been noticed in both model tests and in some calculations, efforts to determine the optimum gap size had assumed that once a small enough gap had been achieved, the nature and magnitude of the loads, including impact loads, would converge to those of a zero gap. Efforts were aimed at finding the point of diminishing returns on an exponential-type either load or bending moment response curve, where forces were determined without consideration for impact loads.
Previous attempts to minimize the gap have been dependent on the tolerances that are achievable in fabricating buoyancy cans, guides, and supporting structures. Recent analytical and model test work has indicated that the conclusions made previously did not fully account for impact loads, and that the nature of the signal is quite different if there is a gap that is large enough for these fabrication tolerances. Loads on the buoyancy can and guide have been found to be large and numerous enough to make practical design for both strength and fatigue difficult. Therefore, there is a need to reduce loads, particularly impact loads, on buoyancy cans.
It has been found that the solution to the above-described problem involves the insertion of an additional flexible element between the guide, the guide support structure, and the buoyancy can. One result of such an insertion is reduction of the effective gap size. In some embodiments of the invention, therefore, the gap will be, effectively, zero, (potentially with some preload). Thus, the insert provides for practical fabrication tolerances. Since the gap size is small, the relative velocity at impact is also small. If the gap is effectively zero, the loads are roughly equivalent to the loads calculated using the closed gap assumption. Additionally, if there were to be an impact load, the stiffness of the connection is reduced, in some embodiments, by designing the compliant guide stiffness to meet load requirements.+
Using a computer simulation program, loads on the guides were computed for a given random excitation for a number of gap sizes both with and without the compliant guide. Results for maximum load from these simulations are shown in FIG. 18. FIG. 18 clearly shows that the maximum loads for a given gap size are reduced tremendously by the insertion of the flexible element, as compared to the previous rigid, steel-to-steel contact designs. FIG. 18 also shows that there is a benefit associated with the use of a preload in some embodiments. However, in alternative embodiments, there is zero preload, since introduction of an unnecessarily high preload could potentially introduce other problems.
According to one example embodiment of the invention, a guide for a buoyancy can on a floating offshore platform is provided. The platform includes at least one support structure adjacent the buoyancy can. The guide comprises at least one compliant guide member supported by the support structure and adjacent the exterior surface of the buoyancy can. Lateral movement of the buoyancy can toward the support structure compresses the compliant member so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage. A wear pad disposed between each guide structure and buoyancy can protects the guide and buoyancy can from friction wear.
According to another example embodiment of the invention, a guide for a buoyancy can on a floating offshore platform is provided. The platform includes at least one support structure adjacent the buoyancy can. The support structure has at least one projection attached thereto. The guide comprises at least one elastomeric compression pad supported by the support structure and adjacent the exterior surface of the buoyancy can. Lateral movement of the buoyancy can toward the support structure compresses the elastomeric compression pad so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage. A wear pad disposed between each elastomeric compression pad and the buoyancy can protects the compression pad from friction wear against the buoyancy can. At least one carriage is attached to the guide. The carriage has a channel therein that slidingly engages the projection on the support structure.
According to still another example embodiment of the invention, a guide for a buoyancy can on a floating offshore platform is provided. The platform includes at least one support structure adjacent the buoyancy can. The support structure has upper and lower projections attached thereto. The guide comprises a plurality of elastomeric compression pads supported by the support structure and adjacent the exterior surface of the buoyancy can. Each compression pad has first and second opposite sides. Lateral movement of the buoyancy can toward the support structure compresses the elastomeric compression pads so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage. A first rigid plate is associated with the first side of the compression pad. A second rigid plate is disposed between and affixed to the support structure and the second side of the compression pad for affixing the compression pad to the support structure. A wear pad support is attached to the first rigid plate. The wear pad support has upper and lower ends and comprises a base plate, a pair of spaced side plates attached to and extending from the base plate, and a top plate extending between the side plates. A wear pad is secured to the wear pad support. The wear pad is disposed between the compression pad and the buoyancy can for protecting the compression pad and buoyancy can from friction wear. Upper and lower carriages extend from the upper and lower ends, respectively, of the wear pad support. Each carriage has a channel therein that slidingly engages a respective projection on the support structure.
According to yet another example embodiment of the invention, a guide for a buoyancy can on a floating offshore platform is provided. The platform includes at least one support structure adjacent the buoyancy can. The support structure has upper and lower projections attached thereto. The guide comprises a plurality of elastomeric compression pads supported by the support structure and adjacent the exterior surface of the buoyancy can. Each compression pad has first and second opposite sides. Lateral movement of the buoyancy can toward the support structure compresses the elastomeric compression pads so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage. A bearing plate is affixed to the first side of the compression pad. A first rigid plate is affixed to the bearing plate. A second rigid plate is disposed between and affixed to the support structure and the second side of the compression pad for affixing the compression pad to the support structure. A wear pad support is attached to the first rigid plate. The wear pad support has upper and lower ends. The wear pad support comprises a base plate, a pair of spaced side plates attached to and extending from the base plate, and a top plate extending between the side plates. A wear pad is secured to the wear pad support. It is disposed between the compression pad and the buoyancy can for protecting the compression pad and buoyancy can from friction wear. Upper and lower carriages extend from the upper and lower ends, respectively, of the wear pad support. Each carriage has a channel therein that slidingly engages a respective said projection on the support structure.
According to still another example embodiment of the invention, apparatus for compliantly guiding a buoyancy can on a floating offshore platform is provided. The apparatus comprises a plurality of spaced support structures attached to the platform and arranged radially around the exterior circumferential surface of the buoyancy can. At least one elastomeric compression pad is attached to each support structure and disposed adjacent the exterior surface of the buoyancy can. Lateral movement of the buoyancy can toward one of the support structures compresses the elastomeric compression pad attached thereto so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage.
According to even a further example embodiment of the invention, for a floating offshore platform having at least one buoyancy can and a support structure adjacent the buoyancy can, a method is provided for protecting the buoyancy can and the support structure from damage caused by impact of the buoyancy can with the support structure. The method comprises supporting at least one compliant member between the buoyancy can and the support structure. The method further comprises absorbing the force generated by lateral movement of the buoyancy can by compressing the compliant member between the buoyancy can and the support structure.
According to still another example embodiment of the invention, for a floating offshore platform having at least one buoyancy can, a support structure for supporting a compliant guide for the buoyancy can is provided. The support structure comprises a T-girder and means for supporting the guide from the support structure.